This work involves the preparation of new, cleavable crosslinking agents which can be used to isolate proteins from animal cell surfaces. These reagents contain a reactive group which can interest or react (or both) with specific cell-surface proteins. Either before or after its interaction with cells, the reagent is attached to a macromolecular support via a cleavable connector arm. Cell-surface components which have become linked to the reagent are selectively removed from the support by cleaving the connector arm. Reagents currently under study contain phenyl ester linkages which are stable under physiological conditions but can be broken quantitatively under mild conditions that do not result in reduction of disulfide bonds or oxidation of glycosyl residues or in the alteration of any of the other groups commonly present in proteins. Initially, the new reagents will be used to characterize the temporal dependence of the interactions of insulin and its receptor. In an effort to learn more about the role of proteolytic enzymes in cellular growth and transformation, we will use these reagents in our studies of protease inhibitors in normal and transformed 3T3 fibroblasts.